Rare earth (RE) elements are a set of fifteen chemical elements in the periodic table, consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). While named rare earths, they are in fact not that rare and are relatively abundant in the Earth's crust with the exception of promethium, which has no stable nuclear isotope.
The rare earth nitrides (RENs) form in the face-centered cubic (FCC) rocksalt NaCl structure with lattice constants ranging from ˜5.3 Å for LaN to ˜4.76 Å for LuN, in total a 5% difference across the series and less than 0.5% between nitrides of neighbouring atomic species. There is clearly potential for epitaxial growth of custom-designed heterostructures, including superlattices, and even for controlled strains to be introduced.
Most of the fifteen RENs are intrinsic ferromagnetic semiconductors with magnetic properties that provide interesting contrasts and promising complementary electronic properties that make them genuinely attractive for spintronic applications. The RENs exhibit a wide variety of hard- and soft-ferromagnetic properties, i.e. the series includes members with small and huge coercive fields. The best example is GdN and SmN; GdN has a coercive field as small as ˜0.01 Tesla, while in contrast SmN has a coercive field in excess of 6 Tesla.
The recent demonstration that this new class of ferromagnetic materials is epitaxy-compatible with group III-nitrides (GaN, AlN and InN), which are a technologically important nonmagnetic semiconductor family for the fabrication of white and blue light emitting diodes and transistors, has raised interest not only for semiconductor-based spintronics but also for the possibility of enhancing the efficiency of GaN-based light emitting diodes.
Group III-nitrides crystallize in either the cubic zinc blende or hexagonal wurtzite structure. Under ambient conditions, the thermodynamically stable structure is the hexagonal wurtzite structure, and commercially available devices such as blue and white LEDs have also a hexagonal wurtzite structure. The wurtzite crystal structure is a member of the hexagonal crystal system or family. Its space group is P63mc in Hermann-Mauguin notation or No. 186 (in the International Union of Crystallography classification).
Success in obtaining REN thin films epitaxially grown on wurtzite (0001) oriented group III-nitride surfaces has been central in obtaining a better understanding of their fundamental properties, in particular demonstrating, for most of them, their intrinsic ferromagnetic semiconducting nature with a wide variety of magnetic properties across the series. GdN and SmN thin films, typically of the order of tens of nanometers in thickness, have been the most studied compounds of the REN series, with several articles published relating to the effect of the growth parameters (growth temperature, RE-nitrogen flux ratio . . . ) on the structural and electronic properties.
Developing heterojunction device structures based on these two nitride families will rely on the understanding and the ability to control, at the atomic scale, the interface structure and chemical stability. Hitherto these aspects have not been studied in depth.
The present disclosure concerns the very first stages of the epitaxial growth of a REN on a group III-nitride material surface, for example GaN, (Al,Ga)N, InN or (In,Ga)N. During such growth, a group III element, (for example gallium) can segregate at the surface during growth of the REN on the group III-nitride surface producing a diluted and complex REN-group III-nitride interface. The absence of a sharp REN-group III-nitride interface deteriorates the structural quality of the REN layer grown on group III-nitride surface. Segregation is equally expected to for epitaxial, polycrystalline and amorphous layers.